Doc/library/queue.rst
!queue --- A synchronized queue class.. module:: queue :synopsis: A synchronized queue class.
Source code: :source:Lib/queue.py
The :mod:!queue module implements multi-producer, multi-consumer queues.
It is especially useful in threaded programming when information must be
exchanged safely between multiple threads. The :class:Queue class in this
module implements all the required locking semantics.
The module implements three types of queue, which differ only in the order in
which the entries are retrieved. In a :abbr:FIFO (first-in, first-out)
queue, the first tasks added are the first retrieved. In a
:abbr:LIFO (last-in, first-out) queue, the most recently added entry is
the first retrieved (operating like a stack). With a priority queue,
the entries are kept sorted (using the :mod:heapq module) and the
lowest valued entry is retrieved first.
Internally, those three types of queues use locks to temporarily block competing threads; however, they are not designed to handle reentrancy within a thread.
In addition, the module implements a "simple"
:abbr:FIFO (first-in, first-out) queue type, :class:SimpleQueue, whose
specific implementation provides additional guarantees
in exchange for the smaller functionality.
The :mod:!queue module defines the following classes and exceptions:
.. class:: Queue(maxsize=0)
Constructor for a :abbr:FIFO (first-in, first-out) queue. maxsize is
an integer that sets the upperbound
limit on the number of items that can be placed in the queue. Insertion will
block once this size has been reached, until queue items are consumed. If
maxsize is less than or equal to zero, the queue size is infinite.
.. class:: LifoQueue(maxsize=0)
Constructor for a :abbr:LIFO (last-in, first-out) queue. maxsize is
an integer that sets the upperbound
limit on the number of items that can be placed in the queue. Insertion will
block once this size has been reached, until queue items are consumed. If
maxsize is less than or equal to zero, the queue size is infinite.
.. class:: PriorityQueue(maxsize=0)
Constructor for a priority queue. maxsize is an integer that sets the upperbound limit on the number of items that can be placed in the queue. Insertion will block once this size has been reached, until queue items are consumed. If maxsize is less than or equal to zero, the queue size is infinite.
The lowest valued entries are retrieved first (the lowest valued entry is the
one that would be returned by min(entries)). A typical pattern for
entries is a tuple in the form: (priority_number, data).
If the data elements are not comparable, the data can be wrapped in a class that ignores the data item and only compares the priority number::
from dataclasses import dataclass, field
from typing import Any
@dataclass(order=True)
class PrioritizedItem:
priority: int
item: Any=field(compare=False)
.. class:: SimpleQueue()
Constructor for an unbounded :abbr:FIFO (first-in, first-out) queue.
Simple queues lack advanced functionality such as task tracking.
.. versionadded:: 3.7
.. exception:: Empty
Exception raised when non-blocking :meth:~Queue.get (or
:meth:~Queue.get_nowait) is called
on a :class:Queue object which is empty.
.. exception:: Full
Exception raised when non-blocking :meth:~Queue.put (or
:meth:~Queue.put_nowait) is called
on a :class:Queue object which is full.
.. exception:: ShutDown
Exception raised when :meth:~Queue.put or :meth:~Queue.get is called on
a :class:Queue object which has been shut down.
.. versionadded:: 3.13
.. _queueobjects:
Queue objects (:class:Queue, :class:LifoQueue, or :class:PriorityQueue)
provide the public methods described below.
.. method:: Queue.qsize()
Return the approximate size of the queue. Note, qsize() > 0 doesn't guarantee that a subsequent get() will not block, nor will qsize() < maxsize guarantee that put() will not block.
.. method:: Queue.empty()
Return True if the queue is empty, False otherwise. If empty()
returns True it doesn't guarantee that a subsequent call to put()
will not block. Similarly, if empty() returns False it doesn't
guarantee that a subsequent call to get() will not block.
.. method:: Queue.full()
Return True if the queue is full, False otherwise. If full()
returns True it doesn't guarantee that a subsequent call to get()
will not block. Similarly, if full() returns False it doesn't
guarantee that a subsequent call to put() will not block.
.. method:: Queue.put(item, block=True, timeout=None)
Put item into the queue. If optional args block is true and timeout is
None (the default), block if necessary until a free slot is available. If
timeout is a positive number, it blocks at most timeout seconds and raises
the :exc:Full exception if no free slot was available within that time.
Otherwise (block is false), put an item on the queue if a free slot is
immediately available, else raise the :exc:Full exception (timeout is
ignored in that case).
Raises :exc:ShutDown if the queue has been shut down.
.. method:: Queue.put_nowait(item)
Equivalent to put(item, block=False).
.. method:: Queue.get(block=True, timeout=None)
Remove and return an item from the queue. If optional args block is true and
timeout is None (the default), block if necessary until an item is available.
If timeout is a positive number, it blocks at most timeout seconds and
raises the :exc:Empty exception if no item was available within that time.
Otherwise (block is false), return an item if one is immediately available,
else raise the :exc:Empty exception (timeout is ignored in that case).
Prior to 3.0 on POSIX systems, and for all versions on Windows, if
block is true and timeout is None, this operation goes into
an uninterruptible wait on an underlying lock. This means that no exceptions
can occur, and in particular a SIGINT will not trigger a :exc:KeyboardInterrupt.
Raises :exc:ShutDown if the queue has been shut down and is empty, or if
the queue has been shut down immediately.
.. method:: Queue.get_nowait()
Equivalent to get(False).
Two methods are offered to support tracking whether enqueued tasks have been fully processed by daemon consumer threads.
.. method:: Queue.task_done()
Indicate that a formerly enqueued task is complete. Used by queue consumer
threads. For each :meth:get used to fetch a task, a subsequent call to
:meth:task_done tells the queue that the processing on the task is complete.
If a :meth:join is currently blocking, it will resume when all items have been
processed (meaning that a :meth:task_done call was received for every item
that had been :meth:put into the queue).
Raises a :exc:ValueError if called more times than there were items placed in
the queue.
.. method:: Queue.join()
Blocks until all items in the queue have been gotten and processed.
The count of unfinished tasks goes up whenever an item is added to the queue.
The count goes down whenever a consumer thread calls :meth:task_done to
indicate that the item was retrieved and all work on it is complete. When the
count of unfinished tasks drops to zero, :meth:join unblocks.
Waiting for task completion ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Example of how to wait for enqueued tasks to be completed::
import threading
import queue
q = queue.Queue()
def worker():
while True:
item = q.get()
print(f'Working on {item}')
print(f'Finished {item}')
q.task_done()
# Turn-on the worker thread.
threading.Thread(target=worker, daemon=True).start()
# Send thirty task requests to the worker.
for item in range(30):
q.put(item)
# Block until all tasks are done.
q.join()
print('All work completed')
Terminating queues ^^^^^^^^^^^^^^^^^^
When no longer needed, :class:Queue objects can be wound down
until empty or terminated immediately with a hard shutdown.
.. method:: Queue.shutdown(immediate=False)
Put a :class:Queue instance into a shutdown mode.
The queue can no longer grow.
Future calls to :meth:~Queue.put raise :exc:ShutDown.
Currently blocked callers of :meth:~Queue.put will be unblocked
and will raise :exc:ShutDown in the formerly blocked thread.
If immediate is false (the default), the queue can be wound
down normally with :meth:~Queue.get calls to extract tasks
that have already been loaded.
And if :meth:~Queue.task_done is called for each remaining task, a
pending :meth:~Queue.join will be unblocked normally.
Once the queue is empty, future calls to :meth:~Queue.get will
raise :exc:ShutDown.
If immediate is true, the queue is terminated immediately.
The queue is drained to be completely empty and the count
of unfinished tasks is reduced by the number of tasks drained.
If unfinished tasks is zero, callers of :meth:~Queue.join
are unblocked. Also, blocked callers of :meth:~Queue.get
are unblocked and will raise :exc:ShutDown because the
queue is empty.
Use caution when using :meth:~Queue.join with immediate set
to true. This unblocks the join even when no work has been done
on the tasks, violating the usual invariant for joining a queue.
.. versionadded:: 3.13
:class:SimpleQueue objects provide the public methods described below.
.. method:: SimpleQueue.qsize()
Return the approximate size of the queue. Note, qsize() > 0 doesn't guarantee that a subsequent get() will not block.
.. method:: SimpleQueue.empty()
Return True if the queue is empty, False otherwise. If empty()
returns False it doesn't guarantee that a subsequent call to get()
will not block.
.. method:: SimpleQueue.put(item, block=True, timeout=None)
Put item into the queue. The method never blocks and always succeeds
(except for potential low-level errors such as failure to allocate memory).
The optional args block and timeout are ignored and only provided
for compatibility with :meth:Queue.put.
.. impl-detail::
This method has a C implementation which is reentrant. That is, a
put() or get() call can be interrupted by another put()
call in the same thread without deadlocking or corrupting internal
state inside the queue. This makes it appropriate for use in
destructors such as __del__ methods or :mod:weakref callbacks.
.. method:: SimpleQueue.put_nowait(item)
Equivalent to put(item, block=False), provided for compatibility with
:meth:Queue.put_nowait.
.. method:: SimpleQueue.get(block=True, timeout=None)
Remove and return an item from the queue. If optional args block is true and
timeout is None (the default), block if necessary until an item is available.
If timeout is a positive number, it blocks at most timeout seconds and
raises the :exc:Empty exception if no item was available within that time.
Otherwise (block is false), return an item if one is immediately available,
else raise the :exc:Empty exception (timeout is ignored in that case).
.. method:: SimpleQueue.get_nowait()
Equivalent to get(False).
.. seealso::
Class :class:multiprocessing.Queue
A queue class for use in a multi-processing (rather than multi-threading)
context.
:class:collections.deque is an alternative implementation of unbounded
queues with fast atomic :meth:~collections.deque.append and
:meth:~collections.deque.popleft operations that do not require locking
and also support indexing.